A nondestructive breakdown phenomenon in selenium rectifiers

Under conditions of excessive current or voltage overload a rectifier will exhibit some form of breakdown. Under practical conditions, breakdown is normally destructive although, with proper safeguards, it may be controlled. A voltage-induced electric breakdown is the type most commonly cited in theory, while in practice a considerable contribution from thermal effects is observed. Selenium rectifiers frequently exhibit the property of recovering from an overvoltage breakdown. The symptoms in this case range from the appearance of minor sparking and fumes of selenium to a very generous arc blowing a cavity in one of the cells. Two physical mechanisms of recovery are apparent; in one, small areas of separation occur between the selenium and the counter-electrode, without necessarily rupturing the latter; in the other, greater burnouts involve craters reaching down to the baseplate, with walls of insulating vitreous selenium, and all counter-electrode removed from the immediate vicinity.